Novel tertiary alcohol-diblocked diisocyanate urea urethane oligomers and coating compositions comprising same

ABSTRACT

This invention provides polymer forming resin systems and high solids solvent based chain-extendable, crosslinkable coating compositions comprising same. The invention comprises a polyepoxide, preferably a diepoxide, of molecular weight between about 100 and about 1000 used generally in amounts from about 10% to about 50% by weight of total resin, and novel diblocked diisocyanate urea urethane oligomers of molecular weight between about 300 and about 5000. The resin components provide chain-extension polymerization during cure at elevated temperature, in situ, on the surface of a substrate. The resin system is self-crosslinking. That is, no additional crosslinking component is required. The cured coatings of the invention provide greatly improved physical properties, in particular greatly improved corrosion resistance.

INTRODUCTION

This invention relates to novel resin systems and to high solids,solvent based coating compositions comprising same. A first componentcomprises any of certain novel chain-extendable, crosslinkable tertiaryalcohol diblocked diisocyanate urea urethane oligomers. The secondcomprises polyepoxide. The resin components provide crosslinking andchain-extension polymerization during cure at elevated temperature, insitu, on the surface of a substrate. The resin system isself-crosslinking, that is, no additional crosslinking agent isrequired. The cured coatings of the invention are highly humidity andsolvent resistant and provide exceptional corrosion resistance.

RELATED APPLICATIONS

This application is related to concurrently filed application Ser. No.334,794, entitled Novel Diblocked Diisocyanate Urea Urethane Oligomersand Coating Compositions Comprising Same; Ser. No. 334,792, entitledNovel Diblocked Diisocyanate Diurea Oligomers and Coating CompositionsComprising Same; and Ser. No. 334,842, entitled Novel TertiaryAlcohol-Diblocked Diisocyanate Diurea Oligomers and Coating CompositionsComprising Same.

BACKGROUND OF THE INVENTION

Solvent based coating compositions are known which employ high molecularweight (e.g. 2,000 to 10,000) polymer resins having crosslinkingfunctionality, and a suitable crosslinking agent. Typically, suchcoating compositions are applied to a substrate, for example, byspraying, and are then cured by baking the coated substrate at anelevated temperature suitable to drive off the organic solvent and topromote the crosslinking reaction. The resulting thermoset coating, ifsufficiently humidity, solvent and corrosion resistant, can provideaesthetic and functional advantages including corrosion protection forthe underlying substrate.

Coating compositions comprising such high molecular weight polymerresins typically comprise only 25% to 50% solids so as to be sprayableor otherwise conveniently applicable to a substrate. The viscosity ofcoating compositions of higher solids content is typically too high forthis purpose. Conventional epoxy ester based automotive vehicle sprayprimers, for example, typically have a volatile organic content ("VOC")of approximately 540 g/l.

Elimination of the volatile organic solvent portion during curing ofthese conventional low-solids coating compositions presents toxicity andin some cases flammability hazards. Furthermore, bulk volume of thesecoating compositions is relatively large and therefore presentsundesirable material handling difficulties, and added expense.Furthermore, excessive solvent losses and/or solvent recovery equipmentadd considerable expense to the coating operation. Recently,governmental regulations on hydrocarbon emissions, particularlyapplicable to automotive coating operations, mandate a significantreduction in volatile organic content for coating compositions. Thus,for example, governmental guidelines for 1982 presently require thatemissions of volatile organics from automotive vehicle primer coatingoperations be reduced to that equivalent to using coating compositionsof no greater than 350 g/l (2.9 lb./gal) VOC. To meet governmentguidelines coating compositions of VOC greater than 350 g/l can beemployed in conjunction with emissions treatment equipment to achievethe specified emissions limit. Such treatment equipment presentssignificant additional expense, however, and thus there is a great needto provide coating compositions of VOC reduced near to, or preferablyeven lower than, the 350 g/l governmental limit.

In response to these concerns, high solids coating compositions havebeen suggested which, typically, employ low molecular weightmulti-functional adducts or copolymers in combination withmulti-functional crosslinking agents. These high solids coatingcompositions are less viscous and, therefore, can be applied byspraying, for example, with far lower VOC than was possible withconventional epoxy ester based coating compositions or otherconventional coating compositions comprising high molecular weightpolymer resins. After application to the substrate, high solids coatingcompositions are cured by baking at a cure temperature, that is, at anelevated temperature suitable to drive off the volatile organic contentand to promote polymerization and crosslinking of the multi-functionallow molecular weight component(s).

Typically, high solids coating compositions yield cured coatings havingpolymeric networks that differ significantly in structure and morphologyfrom the polymeric networks provided by conventional, low solids coatingcompositions comprising high molecular weight polymers. Consequently,the physical properties of the coatings provided by such high solidscoatings compositions can differ significantly from those of the curedcoatings provided by the conventional, low solids coating compositions.In particular, the cured coatings obtained from known high solidscoating compositions can be inferior in that they can be less flexible,less solvent resistant, less adherent to the substrate and/or for otherreasons provide less corrosion inhibition for the underlying substrates.Accordingly, it would be highly desirable to provide a coatingcomposition comprising low molecular weight materials suitable for usein high solids, solvent based coating compositions and yet which, uponcuring, form coatings having physical properties, particularly corrosionresistance, comparable to or better than the physical properties ofcoatings obtained from conventional low solids solvent based coatingcompositions.

Accordingly, it is an object of the present invention to provide novelresin compositions suitable for use in resin compositions suitable foruse in high solids, solvent-based coating compositions. In this regard,it is a particular object of the invention to provide novel coatingcompositions which are curable by chain-extension and crosslinkingduring cure, in situ, on the surface of a substrate to form polymericcoatings similar in properties to those obtainable through use ofconventional low solids, solvent-based coating compositions.

It is a particular object of the invention to provide a coatingcomposition of sufficiently low VOC to facilitate compliance withgovernmental guidelines. It is also an object of the invention toprovide such a coating composition which can be applied to a substrateby spraying or other known method.

It is another object of the invention to provide a method of making acoating on a substrate, which coating has advantageous physicalproperties including humidity, solvent and corrosion resistance.Additional aspects and advantages of the invention will be apparent fromthe following description thereof.

SUMMARY OF THE INVENTION

According to the present invention, low molecular weightchain-extendable, crosslinkable tertiary alcohol-diblocked diisocyanateurea urethane oligomers are provided which are suitable for use in highsolids, organic solvent based coating compositions. The novel oligomersof the invention preferably have number average molecular weight ofabout 300 to about 5000, more preferably about 300 to about 1500 and arethe reaction product of primary or secondary alkanolamine withhalf-blocked diisocyanate. The half-blocked diisocyanate is the reactionproduct of organic diisocyanate with a tertiary alcohol blocking agent.

According to another aspect of the invention, a novel solvent-basedresin composition comprises the novel chain-extendable, crosslinkabletertiary alcohol diblocked diisocyanate urea urethane oligomer of theinvention, polyepoxide bearing preferably about 2 to 10, more preferablyabout 2 to 4 epoxide groups, and having molecular weight of about 100 to1000, more preferably about 300 to 700, and suitable organic solvent.The resin composition comprises latent amine chain-extensionpolymerization functionality and undergoes both chain-extensionpolymerization and crosslinking reaction, in situ, on the surface of thesubstrate during cure of the coating composition to form high molecularweight polyurea polyurethanes. More specifically, the two tertiaryalcohol-blocked isocyanate groups of the novel oligomer of the inventionundergo thermal decomposition at elevated temperature of about 80° C. toabout 220° C., to afford an amine, functional oligomer product, volatilealkene and carbon dioxide. Following such decomposition, the resultingamine functional oligomer product reacts with the epoxy functionality ofthe polyepoxide of the coating composition. That is, during cure of thecoating composition, in situ, on the surface of the substrate, the aminefunctionality generated by the decomposition of the tertiaryalcohol-blocked isocyanate groups will provide chain-extension andcrosslinking reactions with the epoxy groups of the polyepoxidecomponent of the coating composition. Where the polyepoxide bears threeor more epoxy groups, the coating composition, during cure, provides ahigh degree of chain extension polymerization and the cured coating hasa high crosslink density. Where all or a substantial portion of thepolyepoxide is diepoxide, the coating composition, during cure, providesa substantially higher degree of chain-extension and the cured coatinghas a substantially lower crosslink density.

The resin composition of the invention can be formulated into highsolids coating compositions having a viscosity as low as about 40seconds or less, #4 Ford Cup, at 27° C., at calculated VOC of about 400g/l or less.

According to another aspect of the invention, a method of making acorrosion, solvent and humidity resistant coating on a substratecomprises applying to the substrate the novel thermosetting resincomposition of the invention and heating the resin composition tobetween about 80° C. and about 220° C. and preferably to between about110° C. and about 180° C. for a period sufficient to yield a curedcoating. The cured coating, which is yet another aspect of theinvention, is solvent and humidity resistant and has been found toprovide exceptionally good corrosion resistance.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, a high solids coating composition is one comprisingpolymerizable resin in which a volatile organic solvent content of about400 g/l (3.4 lb./gal.) or less yields a viscosity of less thanapproximately 40 sec. #4 Ford Cup at 27° C. (80° F.). Thus, such highsolids coating composition could be applied, for example, by spraytechniques, to a substrate.

The novel crosslinkable, chain-extendable diblocked diisocyanate ureaurethane oligomer component of the invention preferably has a numberaverage molecular weight of about 300 to about 5000, more preferablyabout 300 to about 1500. These oligomers are provided as the reactionproduct of a suitable primary or secondary alkanolamine with suitabletertiary alcohol half-blocked diisocyanate. The hydroxy functionalityand the amine functionality of the alkanolamine each react with the freeisocyanate functionality of a different half-blocked diisocyanatemolecule forming a urethane linkage and a urea linkage, respectively.The reaction product comprises two blocked isocyanate groups, one fromeach of the two half-blocked diisocyanate molecules which reacted withthe alkanolamine. The alkanolamine and half-blocked diisocyanate arereacted together in molar ratio of about 1:2, respectively, according tomethods which are well known to the skilled of the art.

Suitable alkanolamines include primary and secondary alkanolamines ofmolecular weight about 60 to 700, more preferably about 60 to 300. Manysuitable alkanolamines are readily commercially available and will beapparent to the skilled of the art in view of the present disclosure.Preferred alkanolamines include, for example, primary alkanolamines of 2to 20 carbons, for example, ethanolamine, 2-aminopropanol,3-aminopropanol, etc. and secondary alkanolamines, for example,N-alkylalkanolamine, wherein each alkyl moiety and each alkanol moietyhas about 2 to about 20 carbons, for example, N-methylethanolamine,N-ethylethanolamine, N-propylbutanolamine and the like and a mixture ofany of them. Where a high solids coating composition is desired, it isgenerally preferred to use primary and/or secondary alkanolamineswherein each alkyl moiety and each alkanol moiety has only about 2 toabout 6 carbons, since the resulting oligomers of the invention are oflower molecular weight and provide a coating composition of lowerviscosity at a given VOC. Where greater flexibility is desired in thecured coating, it is preferred to use alkanolamines and/orN-alkylalkanolamines having longer chain alkyl moieties, for example, 6carbon to 20 carbon chains.

Suitable half-blocked organic diisocyanate has a number averagemolecular weight of about 120 to about 2000, preferably about 120 toabout 600 and comprises the reaction product of a suitable organicdiisocyanate with suitable monofunctional tertiary alcohol blockingagent in approximately 1:1 molar ratio. Suitable organic diisocyanatesare readily commercially available and include many known to the skilledof the art for example, phenylene diisocyanates, toluene diisocyanates,isophorone diisocyanates, diisocyanatoalkane wherein the alkyl moietyhas preferably about three to about ten carbons, for example,1,4-diisocyanatobutane, 1,6-diisocyanatohexane, and the like or acompatible mixture of any of them. Most preferably the organicdiisocyanate has a molecular weight less than about 250. For lowertemperature curing resin compositions, it generally will be preferredthat the diisocyanate be an aromatic diisocyanate, since the thermaldecomposition temperature of tertiary alcohol-blocked aromaticisocyanate functionality is typically significantly lower that oftertiary alcohol-blocked aliphatic isocyanate functionality.Accordingly, those resin compositions of the invention comprisingtertiary alcohol-diblocked aromatic diisocyanate urea urethane oligomerscure at substantially lower temperature, for example, as low as about80° C. according to preferred embodiments. Generally, it will bepreferred that the organic diisocyanate be selected such that theblocked isocyanate group will remain blocked for long periods of time atnormal storage temperatures, but will be substantially totally"de-blocked" at elevated "cure" temperature. It will be within theskilled of those skilled in the art, in view of the present disclosure,to select a suitable organic diisocyanate to provide de-blockingtemperature meeting the requirements of each particular applicationintended for a coating composition of the invention. It will bepreferred, generally, that the organic diisocyanate be selected suchthat the blocked isocyanate functionality of the diblocked diisocyanateurea urethane oligomer of the invention will undergo thermaldecomposition (i.e., that the coating composition be curable) at atemperature within the range of about 110° to about 180° C.

Suitable half-blocked diisocyanate is prepared by reaction of anysuitable organic diisocyanate, as described above, with sufficientmonofunctional tertiary alcohol blocking agent to block approximatelyone half of the isocyanate functionality. Accordingly, approximately onemolar equivalent of monofunctional tertiary alcohol blocking agent isreacted with approximately one molar equivalent of the organicdiisocyanate. Suitable techniques well known to the skilled of the artcan be employed to maximize the yield of half-blocked diisocyanate, suchas, for example, adding the blocking agent slowly to the organicdiisocyanate under reaction conditions. The half-blocked diisocyanate isthen reacted with the previously described alkanolamine in molar ratioof about 2:1, respectively, to produce the chain-extendable,crosslinkable diblocked diisocyanate urea urethane oligomer of theinvention.

Suitable readily commercially available monofunctional tertiary alcoholblocking agents, that is, alcohols bearing a tertiary hydroxyl group,are well known to the skilled of the art. Preferred monofunctionalblocking agents include those of lower molecular weight, since theblocking agent is lost from the coating composition during the curingprocess and then comprises curing process emissions. Accordingly,blocking agents of 4 to about 20 carbons more preferably 4 to about 6carbons, for example, t-butyl alcohol, 2-methyl-2-hydroxy butane,2-methyl-2-hydroxy pentane, 2-methyl-2-hydroxy hexane, or cyclictertiary alcohols, for example, 1-methylclohexanol and the like or amixture of any of them.

The novel solvent based resin compositions of the invention comprise thenovel chain-extendable crosslinkable tertiary alcohol-diblockeddiisocyanate urea urethane oligomers of the invention and suitablepolyepoxide. Preferred polyepoxides have from 2 to about 10 epoxidegroups, more preferably from 2 to about 4. Preferably the polyepoxide(or each of them) has a number average molecular weight between about100 and 1000, and more preferably between about 300 and 700. Numeroussuitable polyepoxides are readily commercially available and will beapparent to the skilled of the art in view of the present disclosure.Preferred polyepoxides include, for example, any of the wide variety ofacyclic or cyclic aliphatic polyepoxides such as, for example,1,4-butane diol diglycidyl ether, vinylcyclohexene diepoxide andAraldite Cy179 (trademark, Ciba-Geigy Corporation, Ardsley, N.Y.) andaromatic polyepoxides such as, for example, Bisphenol A epichlorohydrinepoxy resins and the like or a compatible mixture of any of them.Preferred polyepoxides include terminal polyepoxides, that is,polyepoxides bearing two terminal epoxide groups, since these aregenerally more reactive and therefore provide coating compositions whichcure faster and/or at lower temperature.

Preferred polyepoxides also include diepoxides. Most preferred in viewof their commercial availability are for example, acyclic and cyclicaliphatic diepoxides such as, for example, 1,4-butanediol diglycidylether and 4-vinylcyclohexene dioxide and aromatic diepoxides such as,for example, Bisphenol A epichlorohydrin epoxy resins, for example, Epon828 and other members of the Epon (trademark) series, Shell ChemicalCompany, Houston, Tex., and DER 331 (trademark) and other members of theDER (trademark) series, Dow Chemical Company, Midland, Mich. Alsopreferred are cycloaliphatic diepoxy resins, for example, the Eponex(trademark) series of Shell Chemical Company, Houston, Tex., hydantoinepoxy resins, for example, Resin XB2793 (trademark), Ciba-GeigyCorporation, Ardsley, N.Y. and epoxy novalak resins such as, forexample, Epon 152 (trademark) and Epon 154 (trademark) of Shell ChemicalCompany. In general, of the above, the lower molecular weight diepoxidesare preferred, for example, Epon 828 (trademark), since a resincomposition of correspondingly lower viscosity (or lower VOC) isprovided. Other, higher molecular weight diepoxides, for example, highermolecular weight members of the Epon (trademark) series, are suitablefor coating compositions of somewhat higher viscosity (or lower solidscontent). The higher molecular weight members of the Epon series, forexample Epon 1001 and Epon 1004, and the like may be somewhat lesspreferred, since they afford coating compositions of higher VOC (orlower solids content). Also, however, improved properties, for example,improved corrosion resistance have been achieved with coatingcompositions comprising such materials and the choice of suitablepolyepoxide will depend upon the particular application intended for thecoating composition.

Polyepoxide can be used in approximately stoichiometric amount in thecoating composition. That is, the coating composition can comprisepolyepoxide and tertiary alcohol-diblocked diisocyanate urea urethaneoligomer in weight ratio of about 1:1 to about 1:10, more preferablyabout 1:1 to about 1:2, respectively. Such composition has been found toprovide efficient chain-extension polymerization during cure of thecoating composition.

Network crosslink density can be controlled, and therefore theflexibility and other properties of the cured coating can to a largeextent be controlled by selection of the molecular weight of thepolyepoxide and of the tertiary alcohol-diblocked diisocyanate ureaurethane oligomer used in the coating composition. Crosslink densityincreases and flexibility decreases as the epoxide equivalent weight ofthe polyepoxide is reduced and/or as the isocyanate equivalent weight ofthe diblocked diisocyanate urea urethane oligomer is reduced. Thus, itwill be apparent to the skilled of the art in view of the presentdisclosure that the selection of the polyepoxide and the selection ofthe organic diisocyanate, blocking agent and alkanolamine reactants forpreparing the diblocked diisocyanates urea urethane oligomer providessubstantial control of the crosslink density in the cured coating. Thus,for example, if the alkanolamine used is ethanolamine, there will be ahigher crosslink density and less flexibility in the cured coating thanif N-hexyloctanolamine is used.

It has been found that those oligomers of the invention comprisingtertiary alcohol-blocked aliphatic isocyanate moieties are more stableand require higher cure temperatures than those oligomers comprisingtertiary alcohol-blocked aromatic isocyanate moieties. Accordingly, curetemperatures of about 110° C. to about 220° C., more typically about150° C. to about 180° C. are required for resin systems of the inventionemploying oligomers comprising blocked aliphatic isocyanate moieties,while temperatures of only about 60° C. to about 150° C., more typicallyabout 80° C. to about 130° C. are required for those employing oligomerscomprising blocked aromatic isocyanate moieties.

Sufficient solvent is used in the coating composition of the inventionto reduce the viscosity of the coating composition to a level suitablefor application to the substrate in the desired manner. The molecularweight of the polyepoxide and of the tertiary alcohol-diblockeddiisocyanate urea urethane oligomer will affect the volatile organiccontent of the coating composition at a desired viscosity. Where ahigh-solids coating composition is desired, preferably lower molecularweight components are employed, since this has been found to providehigh-solids coating compositions which can be applied easily to asubstrate by spray or other means in a coating composition having acalculated volatile organic content of as low as about 350 g/l to 400g/l (2.9 lb./gal. to 3.4 lb./gal.) or less. More specifically, whileconventional epoxy ester-type automotive spray-applied primer coatingcompositions are known to require a volatile organic content of about540 g/l, the novel coating compositions of the present invention havebeen found to require as little as about 350 g/l to 400 g/l (2.9lb./gal. to 3.4 lb./gal.) or less VOC (calculated) to provide aviscosity of less than about 40 sec., #4 Ford Cup at 27° C. (80° F.). Ofcourse, the coating compositions of the invention need not be formulatedas a "high solids" composition, but rather can have a higher VOC toprovide a lower viscosity. It is generally preferred in automotivevehicle spray coating applications and the like, for example, thatsufficient solvent be used to provide a viscosity of about 15 to 40sec., #4 Ford Cup at 27° C. (80° F.).

It will be within the skill of the art to determine the proper volatileorganic content for a given coating composition of the invention for agiven application. In general, suitable solvents include, for example,Cellosolve (trademark), Butyl Cellosolve (trademark), Butyl CellosolveAcetate (trademark), Hexyl Cellosolve (trademark), Hexyl CellosolveAcetate (trademark), Proposol P (trademark), Proposol B (trademark),Proposol M (trademark), all of Union Carbide Corporation, New York,N.Y., butanol, methyl ethyl ketone, methyl amyl ketone and the like, ora compatible mixture of any of them. Additional suitable solvents willbe apparent to the skilled of the art in view of the present disclosure.

Any solvent allowed to remain in the cured coating should be inert so asto avoid adverse effect upon the cured coating or upon another coatingused in conjunction with it during the curing process or thereafter.Preferably the cured coating is completely free of solvent. Thepreferred solvents, in addition, have relatively low volatility attemperatures appreciably below their boiling point such that solventevaporation is low during storage and/or application of the coatingcomposition to the substrate.

Also preferably included in the coating composition of the invention isany of a variety of commercially available catalysts which, in view ofthe present disclosure, will be apparent to the skilled of the art to besuitable to catalyze epoxy/amine reaction. Suitable catalysts include,for example, dibutyl tin dilaurate.

In addition, flow control agent, for example polybutyl acrylate; wettingagent, for example, silicone; pigments; pigment dispersents, corrosioninhibitors, for example, chromate pigments, numerous of all of which areknown to the skilled of the art, may be employed in the coatingcompositions of the invention.

According to another aspect of the invention, a coating on a substrateis provided, which coating comprises the chain-extended, crosslinkedpolymer product following cure of a coating comprising the resin systemof the invention. The coating composition can be a low solidscomposition, that is, it can have a high VOC, but generally a highsolids composition, that is, one having a low VOC, is preferred for thereasons given above. It can be applied by any conventional method,including brushing, dipping, flow coating, spraying, etc. Spraying willgenerally be preferred, or example, for applying the composition as anautomotive vehicle body primer or topcoat. In such sprayingapplications, the coating compositions of the invention are especiallyadvantageous for use as high solids compositions.

Curing the coating composition requires baking for sufficient time atsufficiently elevated temperature to decompose the tertiaryalcohol-blocked isocyanate functionality of the diblocked diisocyanateurea urethane oligomers to generate amine functionality and to promotethe subsequent chain-extension and crosslinking reactions. The time andtemperature required to cure the coating are interrelated and depend, inpart, upon the particular diblocked diisocyanate urea urethane oligomer,polyepoxide, solvent and other materials, if any, used in the coatingcomposition and upon the relative proportion of each. Employing avolatile organic content of about 360 g/l and selecting preferredcomponents as described above, the required bake time and temperature istypically about 20 to 30 minutes at about 150° C. to about 180° C.

It is a significant advantage of the coating composition of theinvention that it does not require the addition of crosslinking agent.Known high solids coating compositions employing melamine crosslinkingagent, for example, are disadvantaged by the cost of same and arefurther disadvantaged by the problems associated with the possibleevolution of formaldehyde during cure of such coating compositions. Itis another advantage of the invention, that the crosslink density can beeasily controlled to a large extent, notwithstanding that a crosslinkingagent is not added to the coating composition of the invention. Asindicated above, the molecular weight and carbon chain lengths of thepolyepoxide and of the reactants for preparation of the diblockeddiisocyanate urea urethane oligomers of the invention can be selected toprovide the desired crosslink density in the cured coating. In addition,the ratio of primary to secondary alkanolamine reactant used to preparethe diblocked diisocyanate urea urethane oligomer affords control of thecrosslink density in the cured coating. In addition, the number of epoxygroups per polyepoxide molecule affords control of the crosslink densityin the cured coatings.

High solids coating compositions according to the present invention,have been found to afford cured coatings with good humidity and solventresistance and with exceptionally good corrosion resistance. Thecorrosion resistance has been found to be comparable and even betterthan that of conventional epoxy ester based, low solids sprayablecoating compositions. The significant reduction in volatile organiccontent provided by the high solids coating composition of the inventioncan be seen to present, therefore, a highly advantageous advance in theart. Thus, for example, cured coatings according to the inventionprovide excellent corrosion resistance when applied over a metallicsubstrate such as, for example, when applied as an automotive vehicleprimer coat over bare sheet steel.

While not wishing to be bound by theory, it is presently understoodthat, upon curing the coating composition of the invention, each of thetwo tertiary carbamate moieties of the diblocked oligomer undergoesthermal decomposition to generate a free amine group through eliminationof carbon dioxide and an alkene. Each of the amine groups of the"de-blocked" or decomposed oligomer is then available to react with theepoxy functionality of the polyepoxide to cure the resin. Morespecifically, each of the two amine groups provide two active hydrogenfor chain-extension and crosslinking reaction with epoxide functionalityof the polyepoxide. Thus, taking t-butanol-blocked oligomer as anexample, it is believed that exposing the resin system to curetemperature would generate two free amine groups on the oligomer throughelimination of isobutylene and carbon dioxide, which free amine wouldthen react with epoxide functionality to form a cured coating comprisinga crosslinked, chain-extended epoxy/amine network. According to thisunderstanding, the cured coating comprises β-hydroxy amine linkages.Since, upon de-blocking, each oligomer provides at least four activehydrogens, (two at each amine functionality) even if the polyepoxideconsists entirely of diepoxide the resin system is self-crosslinking.Hence, it does not require the addition of a crosslinking agent such as,for example, an aminoplast crosslinking agent. It will be appreciated,however, that by varying the epoxide functionality of the polyepoxide orby varying the ratio of diepoxide to higher polyepoxide, the degree ofcrosslinking in the cured coating can be controlled and, hence, to alarge extent the properties of the cured coating, for example,flexability, can be controlled. The selection of polyepoxide(s) willdepend upon the intended application of the particular coatingcomposition.

While not wishing to bound by theory, the exceptional corrosionresistance provided by the invention is believed to stem, in part, fromthe presence in the cured coating of the urea linkage of the oligomerand, in part, from the presence of the C--N bonds of the β-hydroxy aminelinkages formed by chain-extension and crosslinking reaction betweenepoxy functionality of the polyepoxide and free amine functionalitygenerated during curing of the coating composition by the thermaldecomposition of the tertiary alcohol-blocked isocyanate moieties of theoligomers. In addition, the absence of ester linkages in the curedcoating is believed to improve corrosion resistance. Ester linkages areknown to be attacked by hydroxide, a product of the metal corrosionprocess. In contrast, the urea linkages and carbon-nitrogen bonds of thecured coating of the invention are believed to be more highly alkaliresistant and thus highly resistant to degradation processes involvinghydrolysis by cathodicly generated hydroxide, including hydroxidegenerated by the corrosion of a metal substrate. Accordingly, thecoating compositions of the invention provide improved corrosionresistance in comparison to known coating compositions, for example,those comprising only ester or only urethane linkages. Thus, forexample, automotive vehicle primers comprising the coating compositionof the invention and having a calculated volatile organic content of350-390 g/l, have been discovered to provide corrosion resistance farsuperior to that provided by typical low solids, e.g., greater than 500g/l volatile organic content, epoxy ester-based primers.

In addition, the coatings have been found in short term tests to provideexcellent adhesion to substrate, for example, to conversion coated(phosphated) steel, such as that to which an automotive vehicle primercomprising a coating composition according to the invention would beapplied.

It is preferred that the tertiary alcohol-blocked isocyanate groups andtwo of the epoxy groups of the polyepoxide each be an end group.Reactions between such epoxy end groups and de-blocked isocyanate endgroups are believed to provide most efficient chain-extension duringcure.

EXAMPLE I Preparation of Half-blocked Diisocyanate

Blocking agent t-butanol, 74. g, in 10 g methyl amyl ketone is added toisophorone diisocyanate, 222. g, with 0.74 g dibutyl tin dilaurate in 64g methyl amyl ketone, at 35°-45° C. After addition of the alcohol, thereaction temperature is maintained at 35°-45° C. for about 48 hours. Theresulting half-blocked aliphatic diisocyanate was cooled to roomtemperature and stored. The product was characterized by its infraredspectrum (after solvent evaporation) which showed isocyanate absorptionand urethane absorption of approximately equal intensity.

EXAMPLE II Preparation of Diblocked Diisocyanate Urea Urethane Oligomer

Ethanolamine, 30.5 g, is added over a 5-10 minute period to half-blockedisophorone diisocyanate, 380 g, prepared according to Example I. Duringaddition of the ethanolamine, the reaction mixture was maintained at atemperature of 90° C. After addition was completed, the reaction mixturewas stirred for 3 hours, was then diluted with the glycol ether estersolvent Ektasolve E.P. (trademark, Eastman Kodak Company, Rochester,N.Y.), 20. g, and was then stored. The product oligomer was chcterizedby its infrared spectrum which showed a absence of the absorption at2250 cm⁻¹, and the presence of urethane and urea carbonyl absorptions inthe 1670-1710 cm⁻¹ region.

EXAMPLE III Preparation of Coating Composition of the Invention

An automotive vehicle primer comprising a coating composition accordingto the invention is prepared by mixing a pigment package and a resinpackage as follows:

    ______________________________________                                        Pigment Package                                                               Silica                  23.6 g                                                Barytes                 21.4 g                                                Carbon black            0.3 g                                                 Titanium dioxide        5.8 g                                                 Resin Package                                                                 Diblocked diisocyanate                                                        urea urethane oligomer.sup.1                                                                          30. g                                                 Epon 829.sup.2          20. g                                                 Hexyl Cellosolve.sup.3  35. g                                                 Dibutyl tin dilaurate   .4 g                                                  ______________________________________                                         .sup.1 The oligomer is prepared as described in Example II.                   .sup.2 Epon 829 is a trademark of Shell Chemical Company, Houston, Texas,     for Bisphenol A epichlorohydrin epoxy resin.                                  .sup.3 Hexyl Cellosolve is a trademark of Union Carbide Corporation, New      York, New York for organic solvent.                                      

The pigment package is thoroughly dispersed into the binder package. Theresulting composition is ready for use as an automotive primer. It has acalculated volatile organic content of about 370 g/l, and a viscosity ofless than about 40 sec., #4 Ford Cup, at 27° C.

EXAMPLE IV Preparation of Cured Coating

The coating composition of Example III is filtered, sprayed onto Parkercold rolled bare unpolished steel panels and baked at 180° C. for 30minutes. The resulting cured coating has excellent solvent and humidityresistance and shows less than 1 mm scribe line associated adhesion lossafter 24 hours salt spray exposure according to ASTM test method B117using a Singleton SCCH Corrosion test cabinet operated at 35°±2° C.

EXAMPLE V Preparation of Coating Composition of the Invention

An automotive vehicle primer comprising a coating composition of theinvention is prepared by mixing a pigment package and a resin package asfollows:

    ______________________________________                                        Pigment Package                                                               Silica                  23.6 g                                                Barytes                 21.4 g                                                Carbon black            .3 g                                                  Titanium dioxide        5.8 g                                                 Resin Package                                                                 Diblocked diisocyanate                                                        urea urethane oligomer.sup.1                                                                          30. g                                                 Epon 1001.sup.2         40. g                                                 Propasol B.sup.3        60. g                                                 Dibutyl tin dilaurate   .4 g                                                  ______________________________________                                         .sup.1 Prepared as described in Example II.                                   .sup.2 Epon 1001 is a trademark of Shell Chemical Company, Houston, Texas     for a Bisphenol A epichlorohydrin epoxy resin.                                .sup.3 Propasol B is a trademark of Union Carbide Corporation, New York,      New York for organic solvent (nbutoxypropanol).                          

The pigment package is thoroughly dispersed into the binder package. Theresulting composition is ready for use as an automotive primer. It has acalculated volatile organic content of about 370 g/l, and a viscosity ofless than about 40 sec., #4 Ford Cup, at 27° C.

EXAMPLE VI Preparation of Cured Coating

The coating composition of Example V is filtered and sprayed on bare,unpolished steel panels and baked at 180° C. for 30 minutes. Theresulting cured coating has excellent solvent and humidity resistanceand shows less than 0.5 mm of scribe line associated adhesion loss after24 hours salt spray exposure according to ASTM test method B117 using aSingleton SCCH Corrosion test cabinet operated at 35°±2° C.

EXAMPLE VII Preparation of Coating Composition of the Invention

An automotive vehicle primer comprising a coating composition of theinvention is prepared by mixing a pigment package and a resin package asfollows:

    ______________________________________                                        Pigment Package                                                               Silica                  23.6 g                                                Barytes                 21.4 g                                                Carbon black            0.3 g                                                 Titanium dioxide        5.8 g                                                 Resin Package                                                                 Diblocked diisocyanate                                                        urea urethane oligomer.sup.1                                                                          30. g                                                 Eponex 151.sup.2        20. g                                                 Propasol B.sup.3        35. g                                                 Dibutyl tin dilaurate   .5 g                                                  ______________________________________                                         .sup.1 Prepared as described in Example VIII.                                 .sup.2 Eponex 151 is a trademark of Shell Chemical Company, Houston,          Texas, for a cycloaliphatic epoxy resin.                                      .sup.3 Propasol B is a trademark of Union Carbide Corporation, New York,      New York for organic solvent.                                            

The pigment package is thoroughly dispersed into the binder package. Theresulting composition is ready for use as an automotive primer. It has acalculated volatile organic content of about 370 g/l, and a viscosity ofless than about 40 sec., #4 Ford Cup, at 27° C.

EXAMPLE X Preparation of Cured Coating

The coating composition of Example IX is sprayed on bare, unpolishedsteel panels and baked at 180° C. for 30 minutes. The resulting curedcoating has excellent solvent and humidity resistance and shows lessthan 0.5 mm scribeline associated adhesion loss after 24 hours saltspray exposure according to ASTM test method B117 using a Singleton SCCHCorrosion test cabinet operated at 35°±2° C.

Particular embodiments of the present invention described above areillustrative only and do not limit the scope of the invention. It willbe apparent to the skilled of the art in view of the foregoingdisclosure that modifications and substitutions can be made withoutdeparting from the scope of the invention.

What I claim is:
 1. A chain-extendable crosslinkable tertiaryalcohol-diblocked diisocyanate urea urethane oligomer of number averagemolecular weight about 300 to about 5000 comprising the reaction productof primary or secondary alkanolamine of molecular weight about 60 toabout 700, with half-blocked diisocyanate of molecular weight about 120to about 2000 in molar ratio of about 1:2, respectively, whereby boththe hydroxy and amine functionality react with free isocyanate groups toform urethane and urea linkages respectively, said oligomer beingsubstantially free of unreacted hydroxy and amino functionality, saidhalf-blocked diisocyanate comprising the reaction product of an alcoholblocking agent bearing a tertiary hydroxyl group, with organicdiisocyanate in molar ratio of about 1:1, said oligomer having ade-blocking temperature of about 80° C. to about 220° C.
 2. Thechain-extendable crosslinkage tertiary alcohol-diblocked diisocyanateurea urethane oligomer of claim 1 wherein said oligomer has a numberaverage molecular weight of about 300 to about
 1500. 3. Thechain-extendable crosslinkable tertiary alcohol-diblocked diisocyanateurea urethane oligomer of claim 1 wherein said oligomer has ade-blocking temperature of about 110° C. to about 180° C.
 4. Thechain-extendable crosslinkable tertiary alcohol-diblocked diisocyanateurea urethane oligomer of claim 1 wherein said alkanolamine is selectedfrom the group consisting of primary alkanolamines wherein the alkylmoiety has about 2 to about 20 carbons, alkylalkanolamines wherein eachalkyl and alkanol moiety has about 2 to about 20 carbons and a mixtureof any of them.
 5. The chain-extendable crosslinkable tertiaryalcohol-diblocked diisocyanate urea urethane oligomer of claim 1 whereinsaid alkanolamine is selected from the group consisting of ethanolamine,3-aminopropanol, N-methylethanolamine, N-ethylethanolamine,N-propylbutanolamine and a mixture of any of them.
 6. The oligomer ofclaim 1 wherein said tertiary alcohol has about 4 to about 20 carbons.7. The chain-extendable crosslinkable tertiary alcohol-diblockeddiisocyanate urea urethane oligomer of claim 6 wherein said tertiaryalcohol is selected from the group consisting of t-butanol,2-methyl-2-hydroxybutane, 2-methyl-2-hydroxypentane,2-methyl-2-hydroxyhexane, 1-methylcyclohexanol and a mixture of any ofthem.
 8. The chain-extendable crosslinkable tertiary alcohol-diblockeddiisocyanate urea urethane oligomer of claim 1 wherein said organicdiisocyanate is selected from phenylene diisocyanate, toluenediisocyanate, isophorone diisocyanate, diisocyanatoalkane wherein thealkylene moiety has about 3 to about 10 carbons and a mixture of any ofthem.